Cluster Beam Study of (MgSiO3)+-Based Monomeric Silicate Species and Their Interaction with Oxygen: Implications for Interstellar Astrochemistry

Silicates are ubiquitously found as small dust grains throughout the universe. These particles are frequently subject to high-energy processes and subsequent condensation in the interstellar medium (ISM), where they are broken up into many ultrasmall silicate fragments. These abundant molecular-sized silicates likely play an important role in astrochemistry. By approximately mimicking silicate dust grain processing occurring in the diffuse ISM by ablation/cooling of a Mg/Si source material in the presence of O2, we observed the creation of stable clusters based on discrete pyroxene monomers (MgSiO3+), which traditionally have only been considered possible as constituents of bulk silicate materials. Our study suggests that such pyroxene monomer-based clusters could be highly abundant in the ISM from the processing of larger silicate dust grains. A detailed analysis, by infrared multiple-photon dissociation (IR-MPD) spectroscopy and density functional theory (DFT) calculations, reveals the structures and properties of these monomeric silicate species. We find that the clusters interact strongly with oxygen, with some stable cluster isomers having a silicate monomeric core bound to an ozone-like moiety. The general high tendency of these monomeric silicate species to strongly adsorb O2 molecules also suggests that they could be relevant to the observed and unexplained depletion of oxygen in the ISM. We further find clusters where a Mg atom is bound to the MgSiO3 monomer core. These species can be considered as the simplest initial step in monomer-initiated nucleation, indicating that small ionized pyroxenic clusters could also assist in the reformation of larger silicate dust grains in the ISM.

multiple IR photons were absorbed sequentially, leading to a heating of the complex and finally to its fragmentation, and consequently to a depletion of the detected signal in the mass channel of the complex.
The IR-MPD spectra shown in this contribution represent the depletion yield ( ̃) at wavenumber ̃, obtained via the equation where (̃) and 0 are the mass peak intensities with and without laser light, respectively, and (̃) is the macropulse energy. To reduce the IR fluence with which the complexes are irradiated, the whole instrument can be translated up to 300 mm from the focus position leading to a 30-fold reduction in fluence but increased overlap between the laser and molecular beam and thus an increased signal to noise ratio. All spectra presented in this work were recorded 300 mm from the focus. To reduce the IR intensity further, the overlap between laser and molecular beams can be purposely misaligned such that the molecular beam only observes the lower intensity part of the laser beam. Such a reduced IR intensity typically leads to an increased spectral resolution of strong vibrational transitions at the cost of no longer detecting weaker bands. To get the best possible spectral information, IR-MPD spectra were obtained under two different conditions with the molecular beam observing the high and the lower intensity part of the laser beam.

S1.2 Assignment of mass peaks
Laser ablation of a binary MgSi target in the presence of a 1% 16 O2/He pulse results in a typical distribution of magnesium silicate clusters as shown in Figure S1a. This mass spectrum is unusually rich due to the natural isotope distributions of magnesium and silicon, which leads to multiple mass peaks for each cluster size formed. Furthermore, 1% O2 in the He carrier gas results in the preferred production of oxygen-rich clusters (MgxSiyOz + with z > x+y). Reduction of the oxygen percentage only reduces the overall cluster production but does not change the cluster size and composition distribution, showing the necessity of oxygen for the cluster formation process. Due to (1)

S3
with IR-MPD spectra obtained for clusters produced with bare Si and Mg targets, respectively, support or exclude the assignment to SiyOz + and MgxOz + clusters. Based on all these considerations, we were able to unambiguously assign four series of clusters shown in Figure S1: the magnesium oxides MgO6,8,10 + and Mg2O5,7,9 + as well as the magnesium silicates MgSiO5, 7,9,11 + and Mg2SiO7,9,11 + . All other mass peaks cannot be assigned unambiguously and will not be further considered. Figure S1. Ion mass distributions obtained by laser ablation of a Mg:Si = 2:1 target in the presence of a short pulse of helium carrier gas seeded with (a) 1% 16 O2 and (b) 1% 18 O2. The labeled mass signals were unambiguously assigned to the given stoichiometries based on isotope distribution patterns and the IR-MPD spectra.

S1.3 Computational methodology
To obtain different cluster isomers compatible with the experimental stoichiometries (see above) we initially assumed that clusters consist of a cationic bonded silicate core interacting with three oxygen molecules. We obtained candidate low energy isomers for each of the cores (MgSiO3 and Mg2SiO3) using global optimization searches employing the Monte Carlo basin hopping algorithm 5 and a polarizable classical interatomic potential (IP) as detailed in previous studies. 6 For each silicate cationic core structure, we systematically sampled different positions around the core of the three interacting oxygen molecules. The structures of all candidates were optimized using Density Functional Theory (DFT) based calculations with the PBE0 7 hybrid exchange correlation functional, and an accurate tight-tier 2 numerical atom-centered orbital basis set. 8 Non-bonded dispersive interactions were described using the Tkatchenko-Scheffler method. 9 For all structures, harmonic IR spectra were calculated to confirm that S4 our structures correspond to true energy minima and for comparison with the experimental IR spectra.
All DFT calculations were carried out with the Fritz Haber Institute Ab Initio Molecular Simulations package (FHI-AIMS). 10 In addition to species with a core surrounded by separate O2 molecules, we also tested configurations in which one or more oxygen molecules were more intimately interacting with the core and/or with other oxygen molecules. In all cases several spin states were evaluated. We note that in one of the low energy isomers reported we find that the monomeric pyroxene core is so intimately bound to an O2 molecule that it forms a O3 ozone-like moiety (unlike all other reported isomers in which the three O2 molecules remain separate). We further note that DFT calculations using the PBE0 functional have been shown to be suitable for calculating the relative energetics of small oxide clusters interacting with oxygen in various ways with respect to benchmark renormalized second-order perturbation theory calculations. 11 The PBE0 functional was also shown to be suitable for calculating the energetics of O3 interacting/reacting with a small molecule relative to multiconfigurational quantum chemical calculations. 12 In our calculations we assume that the experimentally prepared clusters are cooled enough so that finite temperature anharmonic effects are minimal. Accurate high-level quantum chemical calculations of the neutral bare pyroxenic MgSiO3 monomer have confirmed that anharmonicity hardly affects the pure harmonic IR spectrum. 13 Harmonic DFT calculations using the PBE0 functional have also been shown to accurately reproduce the infrared spectra of bulk crystalline olivine 14 and pyroxene 15 , as well as the spectra of neutral olivinic and pyroxenic monomers. 16 We note that DFT calculations using the PBE0 functional overestimate the frequency of the stretching vibration for the free O2 molecule by ~200 cm -1 . As the O-O stretch vibrations (1400 -1600 cm -1 ) associated with weakly bound O2 molecules are spectrally well separated from the main signature silicate vibrations (300 -1200 cm -1 ), we mainly use the frequencies for the latter calculated using the PBE0 functional to identify experimentally produced isomers. As a separate theoretical check on the high frequency part of the experimental spectra we also performed vibrational calculations on all isomers using the PBE 17 functional which recovers the stretching vibrational frequency of the free O2 molecule relatively well (although being poor for silicate vibrations).

S2.1 Assignment of mass m = 196 amu
One of the most intense signals in the mass spectrum obtained with a mixed MgSi target in the presence of helium seeded with 16 O2 is at mass m = 196 (cf. Figure S1).  Figure S2). The best agreement between the experimental and the calculated isotope distribution is clearly given for MgSi 16 O9 + . To confirm this assignment, the experiments have been repeated with isotopically labeled oxygen, 18 O2 ( Figure   S2,    Figure S2. Due to the significantly different IR-MPD spectra (a) and (b) an assignment to Si3 16 O7 + can be clearly excluded. The blue spectrum has been obtained at reduced IR macropulse intensity. The dots of the experimental spectra represent the sum of typically four to five spectra and the solid lines are obtained by a five-point average.

S2.2 Assignment of mass m = 220 amu
A second intense mass signal in the mass spectrum obtained with a mixed MgSi target in the presence of helium seeded with 16 O2 is at mass m = 220 amu (cf. Figure S1). This mass can in principle be assigned  Figure S4). The best agreement between the experimental and the calculated isotope distribution is given for Mg2Si 16 O9 + , but MgSi3O7 + cannot be completely excluded based on these mass spectra alone. However, the mass spectrum obtained using 18 O2 shows only a very small signal at the mass of MgSi3 18 O7 + , whereas the signal corresponding to the mass of Mg2Si 18 O9 + appears in comparable intensity as m = 220 amu. The calculated isotope distribution of Mg2Si 18 O9 + is also in favorable agreement with the experimentally observed isotope distribution.
Assignment of m = 220 amu to the pure silicate cluster Si5 16 O5 + can be excluded upon comparison of the IR-MPD spectra recorded for m = 220 amu produced with a pure Si target (in this case m = 220 amu can be unambiguously assigned to Si5 16 O5 + ) as shown in Figure S5. The assignment of m = 220 amu to Mg2Si 16 O9 + is confirmed beyond doubt by the similarity of its IR-MPD spectrum in comparison to the one recorded at m = 238 amu with 18 O2. Both spectra (cf. Figure 2 of the main text) show very similar patterns with some spectral shifts as expected to occur due to the different masses of the oxygen isotopes.    MgSiO2 out of plane bending / MgSiO2-SiO3 symmetric stretch (a) Most of the normal modes are coupled modes containing contributions from two or more motions involving several atoms, which makes mode assignments usually difficult. The given mode assignment is based on the most prominent motions, but additional other atoms are typically also involved. (b) two modes with almost the same frequency. OMgO asym stretch (a) Most of the normal modes are coupled modes containing contributions from two or more motions involving several atoms, which makes mode assignments usually difficult. The given mode assignment is based on the most prominent motions but additional other atoms are typically also involved. imentally observed band (ii). The mode labeled (i) can, similarly to the above discussed case of MgSiO9 + , not be explained by fundamental vibrational modes but must instead be assigned to a combination band.    OSiO (of MgSiO2 ring) symmetric stretch b S14 (a) Most of the normal modes are coupled modes containing contributions from two or more motions involving several atoms, which makes mode assignments usually difficult. The given mode assignment is based on the most prominent motions, but additional other atoms are typically also involved.

S4.4 Cartesian coordinates of MgSiO + and Mg2SiO9 + clusters.
Below we include the cartesian coordinates of a selection of low energy clusters from our DFT calculations. The data is given using the xyz file format where the first line gives the number of atoms in the system, the second line gives the relative total energy with respect to the respective lowest energy and the remaining lines provide the optimized cartesian coordinates of each atom.